Conventionally, a martensitic stainless steel pipe has been used widely for such applications as oil wells, and the like because it has high resistance to corrosion for CO2. On the other hand, as for the martensitic stainless steel pipe, quenching cracks develop easily if cooling for quenching in the heat treatment step is all performed by water cooling, because the material thereof has extremely high hardenability. Therefore, to quench the martensitic stainless steel pipe in the heat treatment step, an air cooling method requiring many hours has generally been used, which reduces the manufacturing efficiency.
To eliminate the above-described disadvantage of poor manufacturing efficiency as one purpose, a method described in, for example, WO 2005/035815 (Patent Document 1) has been proposed. In the method described in Patent Document 1, a water cooling method having a high cooling rate and an air cooling method are combined in the temperature range excluding the vicinity of Ms point (a temperature at which martensitic transformation of steel starts in cooling at the quenching time).
Specifically, Patent Document 1 discloses a quenching method in which after being heated and austenitized, a steel pipe is cooled in the order of water cooling, air cooling, and water cooling. Specifically, Patent Document 1 discloses a technique for cooling the steel pipe from the external surface thereof in the water cooling step performed before air cooling so that the cooling rate in the range from 980° C. to point A (680° C. to 350° C.) is 1 to 40° C./sec. After the above-described water cooling step, air cooling is performed so that the cooling rate in the range from point A to point B (30 to 150° C.) is lower than 1° C./sec.